Eyeworm disease in small animals is caused by parasitic infestation of ocular tissues with nematodes such as Thelazia callipaeda and Thelazia californiensis. Fruit flies are the vectors for transmission. Clinical signs vary from mild inflammation and lacrimation to edema, corneal ulceration, conjunctivitis, and even blindness. Control strategies focus on topical or systemic anthelmintic treatments.
Once known as "oriental eyeworm," Thelazia callipaeda is an emergent zoonotic agent that infests the conjunctival sacs of its vertebrate hosts, which include dogs, cats, wild carnivores, wild lagomorphs, and humans, among others. Infestation occurs worldwide in connection with the presence of the vector represented by secretophagous fruit flies of the genus Phortica.
Etiology and Epidemiology of Eyeworm Disease in Small Animals
The causative agents of eyeworm disease in small animals are nematodes of the genus Thelazia. T callipaeda is widely distributed throughout Europe, Asia, and the US. The less common T californiensis is occasionally found in the western US. Both species localize on the surface of the eyes of the definitive hosts (for T callipaeda: dogs, cats, foxes, wolves, martens, badgers, lagomorphs, bears, and coyotes; for T californiensis: dogs, cats, and deer). Both species can infest humans.
Zoophilic fruit flies of the genus Phortica are the intermediate hosts and vectors of T callipaeda. These include P variegata in Europe and P okadai in Asia. Infection is transmitted when male flies harboring third-stage larvae feed on the lacrimal secretions of their hosts (see ). The vector for T californiensis is undetermined.
Thelazia callipaeda can infect the eyes of several animal species, including dogs, cats, rabbits, foxes, racoons, bears, wolves, and humans (1). Adult worms live under the eyelids and nictitating membrane of the host animal (2). After mating, female worms release large numbers of first-stage larvae (3) into the lacrimal secretions of the host. First-stage larvae are then ingested by sucking fruit flies (eg, Phortica variegata) (4) that feed on eye secretions. In the fly (intermediate host), the first-stage larvae develop into second-stage and infective third-stage larvae (5). The third-stage larvae migrate in approximately 21 days to the buccal apparatus of the fly and are released from the proboscis onto the ocular mucosa of a new susceptible host (6).
Courtesy of Professor Jairo Alfonso Mendoza Roldan.
Infection with T callipaeda appears to be spreading throughout the US and Europe. The exception is the UK, where only imported cases have been recorded.
T callipaeda worms are whitish, 7–19 mm long, and move in a rapid serpentine motion across the eye (see ). As many as 100 eyeworms might be evident in the conjunctival sac, in tear ducts, and on the conjunctiva under the nictitating membrane and eyelids.
Heavy Thelazia callipaeda infection in the eye of a dog.
Courtesy of Dr. Domenico Otranto.
Clinical Findings and Diagnosis of Eyeworm Disease in Small Animals
Clinical signs of thelaziasis include excessive lacrimation and epiphora, ocular pruritus, conjunctivitis, keratitis with corneal opacity and ulceration, hyperemia, and rarely, blindness. After local anesthetic, diagnosis is readily accomplished by observing the parasites.
Treatment and Control of Eyeworm Disease in Small Animals
Physical removal of worms
Anthelmintics
After local anesthetic, treatment for eyeworm disease in small animals is readily accomplished by removing the parasites with forceps.
Control strategies focus on topical or systemic anthelmintic treatments; the use of repellents is ineffective against vector flies.
Thelazia spp infections have been successfully eliminated from dogs with administration of milbemycin oxime at a minimum dosage of 0.5 mg/kg, PO (two treatments 1 week apart, in combination with praziquantel) (1); or 2.5% moxidectin spot-on (in combination with 10% imidacloprid), applied once (2). Administration of ocular solutions (1% moxidectin (3) or 2% levamisole) or ointments (1% levamisole or 4% morantel) can also be effective; however, they might cause irritation.
Infection with T callipaeda has also been prevented in dogs with administration of moxidectin at 24 mcg/kg PO, once monthly (in combination with sarolaner and pyrantel) (4); a single dose of sustained-release moxidectin at 0.17 mg/kg, SC (provided 6 months of protection) (5); and with ivermectin, 0.2 mg/kg, PO, every 3 weeks (6).
Milbemycin oxime administered at the minimum dosage of 2 mg/kg, PO (two treatments 1 week apart, in combination with praziquantel) showed high therapeutic efficacy in cats infected with T callipaeda (1).
Key Points
Thelazia callipaeda is a zoonotic eyeworm that infests the conjunctival sacs of dogs, cats, wild carnivores, wild lagomorphs, and humans.
T callipaeda infestation is distributed worldwide and is emergent in the US.
Phortica spp, which are zoophilic fruit flies that feed on the tears of receptive hosts, are vectors of T callipaeda infection.
Various anthelmintic strategies are useful for the treatment and prevention of thelaziasis in dogs and cats.
For More Information
Otranto D, Wall R. Veterinary Parasitology. 5th ed. Wiley; 2024.
Manoj RRS, White H, Young R, et al. Emergence of thelaziosis caused by Thelazia callipaeda in dogs and cats, United States. Emerg Infect Dis. 2024;30(3):591-594.
Otranto D, Mendoza-Roldan JA, Dantas-Torres F. Thelazia callipaeda. Trends Parasitol. 2021;37(3):263-264.
Otranto D, Lia RP, Cantacessi C, et al. Nematode biology and larval development of Thelazia callipaeda (Spirurida, Thelaziidae) in the drosophilid intermediate host in Europe and China. Parasitology. 2005;131(Pt 6):847-855.
Otranto D, Cantacessi C, Dantas-Torres F, et al. The role of wild canids and felids in spreading parasites to dogs and cats in Europe. Part II: Helminths and arthropods. Vet Parasitol. 2015;213(1-2):24-37.
Also see pet owner content regarding eyeworm disease in dogs and cats.
References
Motta B, Schnyder M, Basano FS, et al. Therapeutic efficacy of milbemycin oxime/praziquantel oral formulation (Milbemax®) against in naturally infested dogs and cats. Parasit Vectors. 2012;5:85. doi:10.1186/1756-3305-5-85
Otranto D, Colella V, Crescenzo G, et al. Efficacy of moxidectin 2.5% and imidacloprid 10% in the treatment of ocular thelaziosis by Thelazia callipaeda in naturally infected dogs. Vet Parasitol. 2016;227:118-121. doi:10.1016/j.vetpar.2016.07.035
Lia RP, Traversa D, Agostini A, Otranto D. Field efficacy of moxidectin 1 per cent against Thelazia callipaeda in naturally infected dogs. Vet Rec. 2004;154(5):143-145. doi:10.1136/vr.154.5.143
Bezerra-Santos MA, Mendoza-Roldan JA, Sgroi G, et al. Efficacy of a formulation of sarolaner/moxidectin/pyrantel (Simparica Trio®) for the prevention of canine eyeworm infection. Parasit Vectors. 2022;15(1):370. doi:10.1186/s13071-022-05501-6
Rossi L, Rigano C, Tomio E, Frassetto D, Ferroglio E. Use of sustained-release moxidectin to prevent eyeworm (Thelazia callipaeda) infection in dogs. Vet Rec. 2007;161(24):820-821.
Fudge M, Jeong S, McInturff P. A clinical trial of ivermectin against eyeworms in German Shepherd military working dogs. US Army Med Dep J. 2007;51-56.
